On the screen this makes sense. After all the viewers have to be able to connect with the characters, to understand them and sympathise with them. Also, actors have to play the characters, and if the aliens have a roughly humanoid shape and appearance then their job is made easier.
So we have the latex-forehead brigade of alien races – with a ridge here, a pointy ear there and perhaps some decorative spots all over being enough to persuade us viewers to accept these as alien.
Okay: there are some non-humanoids every now and again; E.T. himself and the animatronic creatures in the Farscape series and Star Wars do come to mind but for the most part aliens in visual science fiction involve latex prosthetics.
But might this be more realistic than we think? Could this convenience brought forth by budgetary constraints actually be accurate to some extent? If we were to encounter alien life exactly what form would it take?
The honest answer has to be "who knows?" The human race has not detected life anywhere in the universe except on Earth. This has not stopped scientists speculating as to what we might find out there.
Carbon Chauvinism is a belief common among exobiologist. As a term it is a little misleading. It does not imply any snobbery – a consideration that life based on elements other than Carbon would be inferior – but that Carbon is the only element with the right balance of chemical stability and reactivity to form and maintain the complex chemicals needed for life.
All life on Earth is based on Carbon. Carbon is a highly reactive element that forms complex molecules easily. Carbon is one of the five elements (together with Hydrogen, Nitrogen, Oxygen and Phosphorus) comprising Deoxyribonucleic Acid, or DNA – the molecules that hold our genetic code.
Carbon is the core element in sugars, proteins and fats. Carbon forms Cellulose and Chitin, the cell walls of plants and the exoskeletons of insects, arachnids and crustaceans. Carbon is found in the haemoglobin molecule in our blood, in our muscles, our nervous system, teeth and bones. Every single part of our bodies and those of every living creature, plant or bacterium on Earth contains Carbon. 18% of the human body is Carbon.
For Carbon to be the basis of life there is another important ingredient required – liquid water. This places a few restrictions on the kind of environments where Carbon-based life might develop, as there is a relatively narrow temperature range in which water can exist as a liquid.
The Earth finds itself at just the right distance from the Sun so that it is not warmed too much, boiling away all the water; or too little, keeping any water the planet possesses locked in the form of ice. We only have to look at Venus and Mars to see the luck we had on this planet.
Planets that are similar to Earth and exist at Earth-like temperatures are likely to provide challenges for life very much like those evolution has overcome on Earth. So Carbon based life on other worlds is likely to be cellular, a form very familiar to us, and one that allows life forms to maintain a balanced, stable chemical environment.
Alien microbial life may well be virtually indistinguishable from terrestrial microbes, and may prove a danger to any Earth-based life that encounters it since we would not have any immunity against its effects.
More complex life forms in these environments are also likely to have very familiar features. Creatures needing to manoeuvre through water are likely to possess an equivalent to fins; flying creatures would still require something akin to wings. The basic form of all life on Earth, bilateral symmetry, can also be assumed as this is the only way of achieving rapid movement in a controllable direction.
That evolution would work in the same way on another planet is a reasonable assumption. We just have to look at our own planet for evidence of this. Eyes have evolved more than once on Earth (human eyes are by no means the best that have evolved) – bat wings evolved separately to bird and Pterosaurian wings.
If we find an Earth-like world out there, it may well be VERY Earth-like. But Carbon is not the only element that might serve as a basis for life.
The next most likely contender for life is Silicon. Silicon is chemically very similar to Carbon, being a member of the same group in the periodic table. But Silicon has a number of factors against its being a basis for life. Silicon atoms are larger and heavier than Carbon atoms, and they do not form double or triple covalent Bonds quite as easily as Carbon.
This handicap can be overcome by including Oxygen in the compounds and having Silicone molecules as the basis for life. But the Silicon-Oxygen bond in Silicones is a very strong bond, and the energy required to break this bond – as would be needed for biochemical reactions – would seem to suggest such a biochemistry would be a non-starter. The Silicon equivalent of Photosynthesis just would not happen.
Also, Silicon Dioxide, the element's equivalent of Carbon Dioxide, is a solid at the temperatures where water is liquid, and a very stable compound. Any equivalent of respiration in Silicon based life forms would have difficulty disposing of the Silicon Dioxide produced.
There are more hurdles for Silicon life to overcome. First, we just have to look at Earth. Although the Earth is relatively Carbon poor and Silicon rich, life here is Carbon based. Also, by analysing the composition of interstellar space it has been found that there are ten times as many Carbon compounds as Silicon compounds – and half of the Silicon compounds we do find also contain Carbon.
Nitrogen, Phosphorus and Sulphur
Nitrogen is another element capable of forming long chain elements at low temperatures, using a liquid solvent such as Ammonia. Compounds with Nitrogen-only backbones, though, are unstable, reverting easily back to elemental Nitrogen.
Phosphorus is also capable of forming long chain molecules, giving it the potential to form the kinds of complex macromolecules that are the building blocks of life, using Phosphine (PH3) as solvent. But Phosphorus is also too reactive for these long chain molecules to be stable.
However, it's when you link these two elements together that things become a little more interesting. Nitrogen-Phosphorus compounds known as Phosphazenes, can form long, relatively stable chain molecules, but the energy required to create them is great – Nitrogen being so much more likely (chemically speaking) to form a bond with other Nitrogen atoms.
Even if such bonds form, Phosphazene chains and rings are only stable in the presence of Oxygen if they have Carbon-based side groups. Phosphorus and Nitrogen are much more likely to exist as molecular Nitrogen and Diphosphorus Pentoxide.
Abundance in the Universe is also a problem. Carbon is much more preferentially formed during the nuclear fusion occurring in Supernovae. So it's not likely that these elements exist in sufficient quantity for life to start.
Suphur is another element capable of producing long-chains, and despite the fact that it too suffers from similar kinds of high reactivity problems as phosphorus and silicon, we do have evidence of life utilising sulphur. On Earth sulphur-reducing bacteria have been discovered, sustaining their existence with the energy from reducing elemental Sulphur to Hydrogen Sulphide. Although these are not true Sulphur-based life forms (they are still Carbon-based) it does show that nature can achieve many things. These bacteria use elemental Sulphur in place of Oxygen in their biochemistry.
Artificial Life forms have long been a staple of science fiction. But just because they are found in fiction doesn't mean we will only find them in fiction. In recent decades, mankind has been building more and more advanced computers and robots. It would seem a reasonable assumption that any other advanced civilisation would also develop Artificial Intelligence.
Life is fragile. We face so many possible catastrophes that may end all life on Earth, from natural disasters and diseases to our own folly. If we advance our levels of technology onwards a few years and then introduce such a disaster we could well leave a self-sustaining machine culture – one that, if sufficiently developed before all life ends, could continue its own mechanical evolution.
Also, given that we have started to send probes into space, venturing beyond the Solar System, it is possible that our first contact with another civilisation may come in encountering their space junk.
If There Is Life, Where Is It?
If we assume that life is common in the Universe then we face the Fermi Paradox. In 1950 physicist Enrico Fermi asked the simple question 'Where Are They?' ‐ if life is common throughout the Universe, why is it we have not encountered it? One possible solution to this is distance. Mankind has only been capable of broadcasting signals outside the Earth's atmosphere for seventy years.
When you compare this with the lifetime of the Earth this is an inconsequential period of time. The Earth and Sun were formed about four and a half billion years ago. It took over a billion years for life to begin in even a very basic way, a further two billion years before the first multi-cellular organisms appeared, and another billion years before animals began appearing on the land surface of our world.
Even after complex creatures covered the whole Earth, intelligence did not automatically follow. Dinosaurs were the dominant species on Earth for tens of millions of years without ever suffering as a result of not having intelligence.
Sixty five million years ago everything began to change. The dinosaurs' day had ended and mammals began to become dominant, leading eventually to our own evolution. We as a species have been around less than half a million years.
It seems reasonable that intelligent life elsewhere in the Universe would also have required such a lengthy period to evolve. But the age of the Earth is a mere third that of the Universe. So could it be possible that life evolved on planets orbiting stars that formed earlier than that of the Sun?
It is possible, but not by all that much in terms of time. Our Sun is a second-generation star. First-generation stars formed from the elements available immediately after the Big Bang, when the Universe contained only the lightest elements, and none of the heavier elements necessary for life. Planets forming around these stars could only be Hydrogen dominated gas giants.
The heavier elements are formed in the massive explosion of a Supernova when a large star reaches the end of its life. These elements seed the gas clouds condensing into forming new stars, and so enabling rocky terrestrial planets to form around these second-generation stars, containing the range of elements required for life to eventually evolve.
This takes time: lengthy periods of time. Stars live for hundreds of millions, if not billions, of years depending upon their size (larger stars die sooner as they use far more of their fuels to maintain their equilibrium).
So for a planet to develop life it must have formed around a second-generation star – at a similar time to our Sun and Earth. This does not give life elsewhere all that much of a head start on us. And if the conditions on that world-out-there did not cause life too many headaches, it's possible that evolution might be happening on a far slower timescale.
We just do not have the ability at the moment to detect life on a small scale. Our search for extraterrestrials has been pretty much restricted to radio and similar signals. Science is moving forward though: the first water detected on an extrasolar Planet was announced in April 2007, so it is likely to be just a matter of time until we have evidence of more complex molecules on worlds out there.
Next, we have to consider distances. The further we look out into the sky the further backwards in time we are looking. Our galactic neighbour, the Andromeda Galaxy, is over two million light years away. We see this Galaxy as it was more than two million years ago. If life evolved there a million years ago and began broadcasting the kinds of signals that can travel the intergalactic distances required for us to detect them, then those signals are still over a million years away from reaching us. Even staying within our own Galaxy, the Milky Way, there are stars 50,000 light years distant, and hence that far in our past.
Even if we allow for civilisations having evolved that far in the past, there is also the sheer vastness of the Universe to consider. The Milky Way is a large Galaxy, containing an estimated 100 billion stars, but it is only one galaxy out of the billions in the Universe. So if another intelligent life form is out there, looking up at their sky and wandering if there is life out there, they too will face the same problem ‐ where do you start to look to find other life-bearing worlds?
Story Copyright © 2007 by I. E. Lester. All rights reserved.
Having worked through all the fiction of Asimov (as well as Heinlein, Clarke, Moorcock, and many others) he moved onto Asimov's non-fiction, encouraging a love of science.
He studied Mathematics and Astrophysics whilst at University and works as a software designer. When not reading sf or factual science, he can often be found watching cricket or rugby, or wandering medieval streets in France or Italy.